The XPE gene of xeroderma pigmentosum, its product and biological roles

DNA Damage and Associated DNA Repair Defects in Disease and Premature Aging

Genetic information is constantly being attacked by intrinsic and extrinsic damaging agents, such as reactive oxygen species, atmospheric radiation, environmental chemicals, and chemotherapeutics. If DNA modifications persist, they can adversely affect the polymerization of DNA or RNA, leading to replication fork collapse or transcription arrest, or can serve as mutagenic templates during nucleic acid synthesis reactions. To combat the deleterious consequences of DNA damage, organisms have developed complex repair networks that remove chemical modifications or aberrant base arrangements and restore the genome to its original state. Not surprisingly, inherited or sporadic defects in DNA repair mechanisms can give rise to cellular outcomes that underlie disease and aging, such as transformation, apoptosis, and senescence. In the review here, we discuss several genetic disorders linked to DNA repair defects, attempting to draw correlations between the nature of the accumulating DNA damage and the pathological endpoints, namely cancer, neurological disease, and premature aging.

DDB2 regulates Epithelial-to-Mesenchymal Transition (EMT) in Oral/Head and Neck Squamous Cell Carcinoma

DDB2 is a sensor of DNA damage and it plays an important role in Global Genomic Repair (GG-NER). Our previous studies show that DDB2 is involved in the regulation of metastasis in colon adenocarcinoma. Squamous Cell Carcinomas in the Oral/Head & Neck region (HNSCC) are particularly aggressive due to high incidence of recurrence and distant metastasis. In this study, we show that DDB2 expression is downregulated in advanced HNSCCs and loss of DDB2 expression coincides with reduced survival. Recent meta-analysis of gene expression data characterized the mesenchymal-type (EMT-type) as one most aggressive cancer cluster in HNSCC. Here, we report that DDB2 constitutively represses mRNA expression of the EMT- regulatory transcription factors SNAIL, ZEB1, and angiogenic factor VEGF in HNSCC cells. As a result, re-expression of DDB2 in metastatic cells reversed EMT with transcriptional upregulation of epithelial marker E-cadherin, and downregulation of mesenchymal markers N-cadherin, Vimentin, and Fibronectin. Interestingly, in a reverse assay, depletion of DDB2 in non-metastatic cells induced expression of the same EMT-regulatory transcription factors. TGFβs are major regulators of Snail and Zeb1, and we observed that DDB2 transcriptionally regulates expression of TGFB2 in HNSCC cells. Re-expression of DDB2 in mouse embryonic fibroblasts (MEFs) isolated from Ddb2 (-/-) knockout-mice resulted in repression of EMT-regulatory factors Zeb1, Snail and Tgfb2. Taken together, these results support the active role of DDB2 as a candidate suppressor of the EMT-process in HNSCC. Early detection leads to significantly higher survival in HNSCC and DDB2 expression in tumors can be a predictor of EMT progression.

Pyruvate kinase M2 interacts with DNA damage-binding protein 2 and reduces cell survival upon UV irradiation

Pyruvate Kinase M2 (PKM2) is highly expressed in many solid tumors and associated with metabolism reprogramming and proliferation of tumors. Here, we report that PKM2 can bind to DNA Damage-Binding Protein 2 (DDB2), which is necessary for global nucleotide excision repair of UV induced DNA damage. The binding is promoted by UV irradiation and K433 acetylation of PKM2. Over expression of PKM2 facilitates phosphorylation of DDB2 and impairs DDB2-DDB1 binding. Furthermore, knocking down of PKM2 increases cell survival upon UV irradiation, while over expression of PKM2 reduces cell survival and over expression of DDB2-DDB1 reverts this effect. These results reveal a previously unknown regulation of PKM2 on DDB2 and provide a possible mechanism for UV induced tumorigenesis.

True lies: the double life of the nucleotide excision repair factors in transcription and DNA repair

Nucleotide excision repair (NER) is a major DNA repair pathway in eukaryotic cells. NER removes structurally diverse lesions such as pyrimidine dimers, arising upon UV irradiation or bulky chemical adducts, arising upon exposure to carcinogens and some chemotherapeutic drugs. NER defects lead to three genetic disorders that result in predisposition to cancers, accelerated aging, neurological and developmental defects. During NER, more than 30 polypeptides cooperate to recognize, incise, and excise a damaged oligonucleotide from the genomic DNA. Recent papers reveal an additional and unexpected role for the NER factors. In the absence of a genotoxic attack, the promoters of RNA polymerases I- and II-dependent genes recruit XPA, XPC, XPG, and XPF to initiate gene expression. A model that includes the growth arrest and DNA damage 45α protein (Gadd45α) and the NER factors, in order to maintain the promoter of active genes under a hypomethylated state, has been proposed but remains controversial. This paper focuses on the double life of the NER factors in DNA repair and transcription and describes the possible roles of these factors in the RNA synthesis process.

Overexpression of DDB2 enhances the sensitivity of human ovarian cancer cells to cisplatin by augmenting cellular apoptosis

Cisplatin is one of the most widely used anticancer agents, displaying activity against a wide variety of tumors. However, development of drug resistance presents a challenging barrier to successful cancer treatment by cisplatin. To understand the mechanism of cisplatin resistance, we investigated the role of damaged DNA binding protein complex subunit 2 (DDB2) in cisplatin-induced cytotoxicity and apoptosis. We show that DDB2 is not required for the repair of cisplatin-induced DNA damage, but can be induced by cisplatin treatment. DDB2-deficient noncancer cells exhibit enhanced resistance to cell growth inhibition and apoptosis induced by cisplatin than cells with fully restored DDB2 function. Moreover, DDB2 expression in cisplatin-resistant ovarian cancer cell line CP70 and MCP2 was lower than their cisplatin-sensitive parental A2780 cells. Overexpression of DDB2 sensitized CP70 cells to cisplatin-induced cytotoxicity and apoptosis via activation of the caspase pathway and downregulation of antiapoptotic Bcl-2 protein. Further analysis indicates that the overexpression of DDB2 in CP70 cells downregulates Bcl-2 expression through decreasing Bcl-2 mRNA level. These results suggest that ovarian cancer cells containing high level of DDB2 become susceptible to cisplatin by undergoing enhanced apoptosis.

DDB2 (damaged-DNA binding protein 2) in nucleotide excision repair and DNA damage response

DDB2 was identified as a protein involved in the Nucleotide Excision Repair (NER), a major DNA repair mechanism that repairs UV damage to prevent accumulation of mutations and tumorigenesis. However, recent studies indicated additional functions of DDB2 in the DNA damage response pathway. Herein, we discuss the proposed mechanisms by which DDB2 activates NER and programmed cell death upon DNA damage through its E3 ligase activity.

Ddb2 is a haploinsufficient tumor suppressor and controls spontaneous germ cell apoptosis

Damage-specific DNA-binding (DDB) protein heterodimer has been extensively studied in the context of nucleotide excision repair. However, the smaller subunit, DDB2, is also implicated in tumor suppressor p53-mediated processes, although the precise details of the DDB2 - p53 interactions are unknown. Here, we report that Ddb2(-/-) and Ddb2(+/-) mice have shortened lifespans and increased frequency and spectrum of spontaneous tumors. Notably, Ddb2 deficiency enhances lung and mammary adenocarcinomas. Ddb2(-/-) mice are smaller than normal. Whereas weights of kidneys and livers are reduced proportionately, spleens from Ddb2(-/-) mice gradually enlarge with age due to lymphoid proliferation. Ddb2(-/-) mice also have larger testes, and the testicular germ cells show significantly decreased spontaneous apoptosis. These changes parallel reduced levels of p53 and its serine 15 phosphorylation in testicular germ cells. Since tumors that appeared in heterozygous Ddb2(+/-) mice conserve the wild-type Ddb2 allele, Ddb2 RNA expression and Ddb2 exon sequence, Ddb2 heterozygosity can facilitate tumor development as a haploinsufficient tumor suppressor. These results demonstrate that in whole animals as in cultured cells Ddb2 can regulate apoptosis and tumor incidence.